Project Summary With improved technologies, overdiagnosis has become a major threat to both health care efficiency and quality of care. Perhaps no condition illustrates overdiagnosis better than thyroid cancer. The incidence of thyroid cancer is rising, with most patients having an excellent prognosis. There is a large reservoir of indolent thyroid cancer, with potential for detecting disease that will not lead to death. We previously showed a marked rise in use of post-diagnosis imaging for thyroid cancer, and we hypothesize that a rise in pre-diagnosis imaging, both thyroid-targeted and imaging associated with incidental cancer discovery, is uncovering malignancies that would never cause harm. The discovery of this low-risk thyroid cancer has the greatest implications for women, who represent close to 75% of all thyroid cancer cases, and older adults, who have the highest thyroid cancer incidence per 100,000, the most growth in incidence, and the greatest risks from subsequent treatments. In this study, we will use Medicare claims data and linked Surveillance, Epidemiology, and End Results (SEER)-Medicare data to determine the contribution of imaging to the diagnosis of thyroid cancer in older adults. In Aim 1, we will determine the relationship between county-level imaging intensity and both thyroid cancer incidence and disease severity. We hypothesize that regional imaging practices will be strong determinants of thyroid cancer incidence. Moreover, we posit that the major determinant of the increase in low-risk disease will be an increase in the use of imaging associated with incidental cancer discovery. In Aim 2, we will determine the drivers of the imaging, including determining the role of physician and patient level factors. Finally, in Aim 3 we will determine the clinical implications of the imaging, specifically population attributable mortality. We anticipate that the rise in cancer incidence secondary to changes in imaging practices will not be associated with population-level improvements in survival. The results of this study will clarify the relationship between imaging, both thyroid-targeted and imaging associated with incidental cancer discovery, and the rise in thyroid cancer incidence. These findings will be used to develop a clinical strategy to minimize the overdiagnosis of thyroid cancer and to serve as a model for overdiagnosis in general.